There are disclosed a number of fluid-filled insoles, for example in European Patent EP1891869 and U.S. Pat. Nos. 6,665,959, 6,865,823, 7,013,584 and 7,243,446 by Vindriis. These insoles are made of a relatively soft material. The reason for using soft highly elastic plastic foils is that these foils by means of the hydraulic pressure of the liquid allow the insoles to form a large surface against the foot, so as to maximize comfort and pain relief.
In order to increase the ability of insoles to distribute the weight evenly across the foot, it is proposed in U.S. Pat. Nos. 5,878,510, 6,138,382, and 6,178,663 by Schoesler to use a highly viscous liquid and constricted passages for achieving a slow flow of the liquid in the insole. Water is explicitly disclosed as considered unsuitable for such insoles. As material for the insole, polyurethane is preferred, although, other materials are considered useful, such as EVA (ethylene vinyl acetate), polyvinyl chloride (PVC) or vinyl.
A viscous liquid with a viscosity between water and honey in an insole is proposed in German patent publication DE19522100 by Jürgens. As a preferred material for the foil of the insole, soft weldable PVC is disclosed.
Shock absorption is also the objective in International patent application WO2013/006393, which, however, proposes a micro-particulate medium in a reservoir that is attached to a sole made of polymer foam.
In all the above insoles, the distribution of load from one to another part of the foot is the primary objective, why the sole materials are selected as relatively soft highly-elastic plastic foils. Moreover, soft polymer has the further advantage that it is suitable for high-frequency welding, which is a preferred method of attaching the foils to one another along the edge.
Although, the above soles are good for weight distribution generally by distributing foot load over a larger area, the effect on nerve stimulation for improving the ability for balance is very small. Loss of balance ability, however, is a very big problem in connection with a number of diseases such as multiple sclerosis, Parkinson's disease, diabetes, heart stroke, etc., but also in relation to the possibility of elderly to help themselves. It is presumed that an improvement of the balance ability improves the conditions in the above diseases. In this connection, it must be recalled that foot injuries are very common, especially, in diabetes, and the total costs for treatment are considerable.
It is therefore desirable to provide new means for increasing balance.